High frequency transformer assembly



June 30, 1959 F. A. WOOD HIGH FREQUENCY TRANSFORMER ASSEMBLY Filed Jul},' 20, 1954 FRAN K A. WOOD INVENTOR %W% HIS ATTORNEY.

United States atent 1::

HIGH FREQUENCY TRANSFGRMER ASSEMBLY Frank A. Wood, Elmhurst, Ill., assignor to Zenith Radio Corporation, a corporation of Delaware Application July 20, 1954, Serial No. 444,468

8 Claims. ((31. 333-78) This invention relates to the structural features of a tuned high-frequency transformer which may be used, for example, as a frequency selective coupling network in the intermediate-frequency portion of a superheterodyne receiver. When so used, it is usually referred to as an intermediate frequency or IF transformer assembly.

Generally, IF transformers are two-winding structures having a primary and a secondary winding and a pair of tuning condensers each of which is connected in shunt relation with one of the transformer windings. In the usual construction the windings are secured to a coil form while the condensers are mounted on a terminal board disposed in perpendicular relation with respect to the coil form. More specifically, each condenser is in the nature of a subassembly, being constructed of a single sheet of mica dielectric having opposed silvered areas on obverse sides thereof which areas define the electrodes of the condenser. The transformer may be tuned over a range of operating frequencies by tuning adjustments which are often times cup-like externally threaded cores positioned at opposite ends of the coil form and adjustable axially thereof so that as each core moves axially with respect to the coil form the appareut inductance of the associated transformer winding is varied.

It has been found that this use of cup-like tuning cores in IF transformers does not achieve the quality factor or improvement commensurate with the amount of core material that is required and this results, at least in part, because such transformers are fundamentally devices of low current density. Expressed in other words, such prior arrangements do not achieve the efficiency exhibited by structures embodying the present invention. Moreover, in the usual prior art structure it is not conveniently possible to adjust both tuning cores from the same end of the transformer assembly; rather, a single core is accessible from one end of the structure while the other is adjustable only from the opposite end thereof. Also, where the subassembly constituting the condensers is disposed perpendicularly of the coil form, the transformer itself is objectionably large in cross-section.

Efforts have been made to increase the efiiciency of the described IF transformer assemblies; notably, magnetic shields of low loss magnetic material have been employed to decrease the reluctance of the flux paths and achieve an over-all increase in quality factor or Q. Most frequently, prior art magnetic shields are formed of comminuted ferromagnetic material which has been molded or compressed into the desired configuration. This type of core is relatively costly to fabricate but, more significantly, is undesirably brittle and subject to breakage.

It is an object of the present invention therefore to provide a new and improved transformer assembly suitable for use as an IF transformer and having an improved magnetic shield.

It is a further object of the invention to provide an intermediate-frequency transformer characterized by a 2,892,985 Patented June 30, 1959 novel physical structure and exhibiting a high quality factor.

It is still another object of the invention to provide an improved magnetic shield for an IF transformer which is compact, highly efiicient and mechanically strong or rugged.

In accordance with the present invention, an intermediate-frequency transformer assembly comprises a supporting frame to which is affixed a first ferromagnetic core structure. That structure comprises a core member, having a nonlinear principal axis, fixed to said frame and having its opposed axial extremities disposed along a line which is parallel to a first predetermined axis and it further comprises a tuning member which is supported for movement along the aforesaid first axis contiguous to the aligned extremities of the nonlinear member. A primary winding of the transformer is mounted on the nonlinear core member. The transformer has a second ferromagnetic core structure of similar construction, likewise comprising a core member, having a nonlinear principal axis, secured to the supporting frame with its opposed axial extremities disposed along a line parallel to a second axis and a tuning member supported for movement along the aforesaid second axis contiguous to the extremities thereof. The second winding of the transformer is disposed about the second core structure in inductive coupling relation to the first winding. Tuning or resonating of the windings is accomplished by a pair of condensers individually constituted by a flat plate of insulating material having a pair of conductive electrodes on opposite or opposed surfaces thereof and each condenser assembly is likewise affixed to the supporting frame with the plane of the insulating plate parallel to that of the supporting frame. The condensers are electrically coupled to the windings through suitable electrical means coupling the first condenser in parallel with the primary winding and coupling the other in parallel with the secondary winding.

A magnetic shield for the transformer assembly is also provided in accordance with another feature of the invention. It includes a first hollow container, popularly referred to as a shield can. The intermediate-frequency transformer is positioned within the shield can and a second or further magnetic shield is interposed between the transformer and the shield can. The second shield member is a hollow thin-walled component of pliable insulating material and is generally cylindrical in configuration and impregnated with comminuted ferromagnetic material.

The features of the present invention which are be lieved to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals indicate like elements, and in which:

Figure 1 is a sectional view of a shielded transformer assembly constructed in accordance with the invention for use as an IF transformer;

Figure 2 is a further sectional view taken along the line 2-2 of Figure 1;

Figure 3 is an exploded perspective view illustrating certain of the components which make up the composite transformer assembly; and

Figure 4 is a view looking into the bottom of the container which also serves as a shield for the transformer.

In describing the structural features of the transformer, it is convenient initially to make reference to the exploded view of Figure 3 representing the details of certain component parts of the assembly. The transformer is composed of two principal subassemblics which are essentially duplicates of one another and it is sutficient therefore to consider the details of but one.

The first subassembly includes a supporting frame 11 which may be a formed or molded piece of plastic or other well-known insulating material. On one face of the frame member there is a discontinuous raised section 12 of L-shaped configuration which is provided for the purpose of locating and securing the core structure of one Winding of the transformer as will be made more clear hereinafter. The discontinuity of the formed section 12 results from an aperture 13 provided to receive the peripheral section of the winding supported on the core structure to minimize the thickness dimension of the subassembly. Opposite raised section 12 on the same side of the frame member is a trough or recess 14 which is threaded to receive an externally threaded tuning memher to be considered more particularly hereinafter. A peripheral slot 15 in support 11, adjacent threaded recess 14, is to accommodate a clamp for securing the tuning member in nested relation with recess 14. The series of slots 17-21 and the additional peripheral notches 22, 22 permit mechanically associating and interlocking terminal elements with support 11 in a manner to be described. The remaining apertures 23 are to receive eyelets or small machine bolts employed in fabricating the final assembly of the transformer while the projection 24 extending from the upper edge of the frame constitutes a locking projection for securing the frame Within a shield can as will be made clear presently.

The core structure to be mounted on frame 11 is shown at 30 of Figure 1. It is a ferromagnetic member having a nonlinear configuration which, for convenience, is shown as an Lshape although it may be semicircular, crescentlike or any other configuration presenting terminal extremities which border upon a common line or plane which parallels the axis of movement of the tuning core. The longer arm of the L-shaped core carries the primary winding 31 of the transformer and the core with its winding may be assemled on frame 11 by nesting the core within the L-shaped raised portion 12 with the peripheral portion of winding 31 received within recess 13. This relationship may be maintained through the use of glue or other suitable adhesive. The tuning core 32 also appears in Figure 1. It is a straight or linear member bearing an external thread and is nested within threaded recess 14 of frame 11, being retained therein by means of a U-shaped clamp 33 received in peripheral slot 15 of the frame member and extending from one side of the frame, through the slot and across tuning member 32. The position of recess 14 in respect of the physical dimensions of L-shaped core 30 causes tuning member 32 effectively to bridge the terminal extremities of the L-shaped core to complete a magnetic circuit for primary winding 31. The threaded engagement of tuning member 32. With recess 14 permits axial movement of the tuning member thereby to provide an adjust-able air gap within the magnetic circuit as the tuning member moves downwardly past the upper extrernity of core member 30. This, of course, varies the apparent or effective inductance of the primary winding in known fashion. Adjustment of tuning member 32 rela tive to the terminals of core portion 30 is accomplished by a slot provided in the upper end of member 32 to receive a screwdriver. Both core members 30 and 32 are preferably constructed of ferrites, a low loss magnetic material which. is molded to the desired configuration and fired.

The remaining principal subassembly, as indicated above, is of identical construction and components thereof which correspond to the components of the subassembly including frame 11 are identified by like reference numerals primed. The winding 31', which appears clearly in Figure 2, is supported by frame 11' and may be considered to constitute the secondary winding of the transformer.

It is usual practice in fabricating IF transformer assemblies to provide fixed condensers to be connected individually across the primary and secondary windings and the mechanical structure under consideration facilities the inclusion of such condensers while, at the same time, retaining a desirably small physical size. To that end, a third subassembly is arranged between the subassemblies including frames 11 and 11.

The third subassembly comprises a single sheet 40 of mica dielectric which has a pair of silvered areas 41 and 42. on one face and a matching pair of silvered areas (not shown) on the obverse face, of identical configuration and in alignment with corresponding ones of the first mentioned areas 41 and 42. The silvered areas constitute electrodes of the condensers as is Well understood. On one face of the sheet 40 there is a further silvered area 43 through which is provided a mounting hole 44. In like fashion at the other end of sheet 40 there is a silvered area 43 on the opposite surface of the sheet and in the immediate vicinity of a second mounting hole 44. Terminal connectors 45, 46 and 47 are associated with the silvered areas on one side of sheet 40 and generally similar connectors 45', 46 and 47' are associated with the corresponding silvered areas on the opposite surface of the sheet. Each connector is bent back upon itself at one extremity to provide a portion such as that designated 45a for entering a slot in one of the supporting frames to anchor the terminal in position. Each connector also has an offset such as that designated 45b to be received in a recess 22 or 22 of the associated supporting frame further to clamp the connector in position within the assembly. Certain of the connectors have an intermediate terminal portion, such as that designated 46c, which is exposed when the terminal is in position projecting through its supporting frame, to receive a circuit connection.

In assembling the major components 11, 11 and 40 the several terminals 4547 and 45'-47 are placed in position by inserting their several projections through the appropriate slots of the frame members and homing their offset sections in the assigned one of the cutouts 22 or 22'. For example, terminal portion 450 of terminal 45 is introduced into slot 17 and its offset 45b by engaging a cutaway 22 locks the terminal in position on frame 11. The next terminal 46 is positioned by inserting its uppermost bent-over section into slot 18 while its intermediate portion 460 is received in slot 21 and its offset received in recess 22. It should be noted that interme dia-te section 470 of connector 47 faces away from frame 11. It passes through a slot in plate 40 to enter a slot 20' and be available at the front face of frame 11. In similar fashion intermediate portion 450 of connector 45 supported by frame 11' projects through an opening in plate 40 into slot 20 to be available at the front face of frame 11.

When the several terminal connectors are in position, dielectric plate 40 is in place between the frame members which are effectively superposed in back to back relation. The recesses 13 and 13' accommodating the primary and secondary windings 31, 31' maintain those windings in inductive coupling relation. The stack or composite assembly is locked by mounting bolts or eyelets which pass through apertures 23 of frame 11, apertures 44 of dielectric plate 40 andv apertures 23' of frame 11. The positioning of the several parts is such that connector 46 is in electrical circuit engagement with condenser electrode 42 and its intermediate portion 466 is available on the free surface of frame 11. Connector 45 is in circuit engagement with the corresponding condenser electrode and its intermediate portion 450' is also available at the same face of frame 11 so that the free ends of the primary winding may be connected to terminal sections 460 and 45c'-which couples the first condenser in parallel relation with the primary winding.

In like manner connector 46 is in circuit engagement with the condenser electrode opposite surface 41 and its intermediate portion 46c is available at the free face of frame 11'. Connector 47 which is in circuit engagement with the remaining electrode 41 of that same condenser presents the free end of its intermediate portion 47c at the same face of frame 11' so that connections may be made to the second winding 31. This connects the second condenser in parallel relation with or across the secondary winding.

The remaining two connectors 45 and 47 are not associated with any of the condensers thus far described. They are merely free connectors which are provided because when the transformer assembly is to be employed in a transistorized receiver, one wherein the transformer constitutes the interstage coupling of transistor type amplifiers, it is convenient to bring out a tap from each winding. The tap of the primary winding is connected to the free terminal of connector 45 and a corresponding tap of the secondary winding is connected to the free terminal of connector 47'.

In order to increase'the efiiciency of the described transformer it is expedient to position it within a shielding container. A suitable container 50 is shown in Figgures l and 2. It is rectangular in cross-section and so dimensioned that the transformer when inserted along a diagonal fits snugly therein. A pair of apertures 51 and 52 are provided in the top surface of the shield in alignment with the slotted ends of adjustable cores 32 and 32'. Such apertures facilitate inserting a screwdriver or similar tool into the slot of the tuning cores for the purpose of effecting adjustments. The top peripheral projections 24 and 24' of frame members 11 and 11' are received in a third aperture 53 of the shield to retain the assembly in its preferred position.

In certain embodiments of the invention it is convenient to introduce an additional shielding member 54 positioned around the transformer assembly, between that assembly and the shield container 50. It preferably is a generally rectangularly shaped thin-walled pasteboard envelope which has been impregnated with a mixture of high permeability powdered iron and powdered ferrite. Alternatively, the separate intermediate shielding element 54 may be omitted and its function achieved by coating the inner surface of shielding container 50 with the same type of mixture of high permeability powdered iron and powdered ferrite. Where that alternative is adopted it is convenient to strike out locking or clamping projections 55 and 56 within the shield as shown in Figure 4. These projections engage opposite faces of the transformer assembly and hold it firmly in position within the shield.

A suitable mixture for impregnating shield element 54 or for coating the inner surface of the shield container 50 comprises two parts of high permeability powdered iron such as that known commercially as Carbonyl to one part powdered ferrite with a suitable binder such as varnish. A coating of this mixture may be applied by spraying or dipping and subsequently baking.

The described transformer, as indicated above, is particularly suited as a frequency selective interstage coupling network for a radio receiver employing stages of amplification or frequency changing which feature the use ofthree electrode transistors. Physically the transformer is extremely small and has been constructed, for example, with frame members 11 and 11 one inch by one inch by one-sixteenth inch. At the same time, the transformer presents terminals to facilitate making individual connections with the three electrodes of the transistor and it exhibits a highly desirable quality factor or Q. Both primary and secondary circuits of the transformer may be tuned from the same end of the assembly and in a most simple manner by axial displacement of tuning member 32 or 32' to control the effective air gap in the magnetic circuit of either winding.

The structure is further characterized by the fact that the tuning condensers are disposed in generally the same plane as the supporting frames of the windings so that the thickness dimension of the assembly is minimized.

The shield element 50 is quite inexpensive to manufacture and overcomes the brittleness of earlier structures. Consequently the shield assembly is much stronger and rugged in a mechanical sense.

While a particlar embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An intermediate-frequency transformer comprising: a supporting frame; a first ferromagnetic core structure comprising an L-shaped core member fixed to said frame with its extremities disposed along a line parallel to a first predetermined axis, and further comprising a tuning member of linear configuration movable along said axis and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of said members; a primary winding carried on said core member; a second ferromagnetic core structure comprising an L-shaped core member fixed to said frame with its extremities disposed along a line parallel to a second predetermined axis, and further comprising a tuning member of linear configuration movable along said second axis and bridged between said extremities of the last-mentioned core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a secondary winding carried on said last-mentioned core member and inductively coupled to said primary winding; a pair of capacitors, each comprising a flat plate of insulating material provided with a pair of conductive electrodes disposed on opposite surfaces of said insulating plate, supported by said frame with the insulating plate thereof disposed in parallel relation to said frame; means electrically coupling one of said condensers in parallel with said primary winding; and means electrically coupling the other of said condensers in parallel with said secondary winding.

2. An intern1ediate-frequency transformer comprising: a supporting frame having a pair of coil-receiving apertures and a pair of positioning portions of nonlinear configuration projecting from opposite faces thereof; a first ferromagnetic core structure comprising a core member fixed on said frame in nested relation with one of said nonlinear positioning portions with its extremities disposed along a line parallel to a first predetermined axis, and further comprising a tuning member of linear configuration movable along said axis and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a primary winding carried on said core member with a peripheral portion thereof received in one of said apertures of said frame; a second ferromagnetic core structure comprising a core member fixed on said frame in nested relation with the other of said nonlinear positioning portions with its extremities disposed along a line parallel to a second predetermined axis, and further comprising a tuning member of linear configuration movable along said second axis and bridged between said extremities of the last-mentioned core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the rela tive positions of such members; a secondary winding carried on said last-mentioned core member with a pcripheral portion thereof received in the other of said apertures of said frame and in inductive coupling relation to said primary winding; a pair of capacitors, each comprising a flat plate of insulating material provided with a pair of conductive electrodes disposed on opposite surfaces of said insulating plate, supported by said frame with the insulating plate thereof disposed in parallel relation to said frame; means electrically coupling one of said condensers in parallel with said primary winding; and means electrically coupling the other of said condensers in parallel with said secondary winding.

3. An intermediate-frequency transformer comprising: a supporting frame having a pair of coil receiving apertures, a pair of positioning portions of nonlinear configuration projecting from opposite faces thereof, and a pair of threaded recesses individually in space opposed relation to one of the positioning portions; a first ferromagnetic core structure comprising a core member fixed on said frame in nested relation with one of said nonlinear positioning portions with its extremities disposed along a line parallel to the axis of one of said threaded recesses, and further comprising an externally threaded tuning member of linear configuration received in said one recess and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a primary winding carried on said core member with a peripheral portion thereof received in one of said apertures of said frame; a second ferromagnetic core structure comprising a core member of nonlinear configuration fixed on said frame in nested relation with the other of said nonlinear positioning portions with its extremities disposed along a line parallel to the axis of the other of said threaded recesses, and further comprising an externally threaded tuning member of linear configuration received in said other recess and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of said members; a secondary Winding carried on said last-mentioned core member with a peripheral portion thereof received in the other of said apertures of said frame and in inductive coupling relation to said primary winding; a pair of capacitors, each comprising a flat plate of insulating material provided with a pair of conductive electrodes disposed on opposite surfaces of said insulating plate, supported by said frame with the insulating plate thereof disposed in parallel relation to said frame; means electrically coupling one of said condensers in parallel with said primary winding; and means electrically coupling the other of said condensers in parallel with said secondary winding.

4. An intermediatefrequency transformer comprising: a frame including a pair of back-toback supports individually having a coil-receiving aperture and a positioning portion of nonlinear configuration projecting from one face thereof; a first ferromagnetic core structure comprising a core member fixed on said frame in nested relation with one of said nonlinear positioning portions with its extremities disposed along a line parallel to a first predetermined axis, and further comprising a tuning member of linear configuration movable along said axis and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the: relative positions of such members; a primary winding carried on said core member with a peripheral portion thereof received in one of said apertures of said frame; a second ferromagnetic core structure comprising a core member fixed on said frame in nested relation with the other of said nonlinear positioning portions with its extremities disposed along a line parallel to a second predetermined axis, and further comprising a tuning member of linear configuration movable along said second axis and bridged between said extremities of the lastmentioned core member to constitute therewith a singleloop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a secondary winding carried on said last-mentioned core member with a peripheral portion thereof received in the other of said apertures of said frame and in inductive coupling relation to said primary winding; a pair of condensers, each comprising a flat plate of insulating material having conductive electrodes on opposed surfaces thereof, carried by said frame intermediate said pair of supports with the insulating plate thereof in parallel relation to said supports; means electrically coupling one of said condensers in parallel with said primary winding; and means electrically coupling the other of said condensers in parallel with said secondary winding.

5. An intermediate-frequency transformer comprising: a supporting frame having a pair of coil-receiving apertures and a pair of positioning portions of nonlinear configuration projecting from opposite faces thereof; a first ferromagnetic core structure comprising a core member fixed on said frame in nested relation with one of said nonlinear positioning portions with its extremities disposed along a line parallel to a first predetermined axis, and further comprising a tuning member of linear config uration movable along said axis and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a primary winding carried on said core memher with a peripheral portion thereof received in one of said apertures of said frame; a second ferromagnetic core structure comprising a core member fixed on said frame in nested relation with the other of said nonlinear positioning portions with its extremities disposed along a line parallel to a second predetermined axis, and further comprising a tuning member of linear configuration movable along said second axis and bridged between said extremities of the last-mentioned core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a secondary winding carried on said last-mentioned core member with a peripheral portion thereof received in the other of said apertures of said frame and in inductive coupling relation to said primary winding; a pair of capacitors, each comprising a flat plate of insulating material provided with a pair of conductive electrodes disposed on opposite surfaces of said insulating plate, supported by said frame with the insulating plate thereof disposed in parallel relation to said frame; means electrically coupling one of said condensers in parallel with said primary winding; means electrically coupling the other of said condensers in parallel with said secondary Winding; a hollow shield container housing said supporting frame; and a magnetic shield interposed between said frame and said container and comprised of a hollow thin-walled member of insulating material impregnated with a comminuted ferromagnetic composition.

6. An intermediate-frequency transformer comprising: a supporting frame; a first single loop ferromagnetic core structure comprising a core member, having a non-linear principal axis, fixed to said frame, the axial extremities of said core member being disposed along a line parallel to a first predetermined axis, and further comprising a tuning member supported for movement along said first axis contiguous to said extremities of said core member; a primary winding carried on said core member; a second single-loop ferromagnetic core structure comprising a core member, having a nonlinear principal axis, fixed to said frame, the axial extremities of said last-mentioned core member being disposed along a line parallel to a second predetermined axis, and further comprising a tuning member supported for movement along said second axis contiguous to said extremities of said last-mentioned core member; a secondary winding carried on said lastmentioned core member and inductively coupled to said primary winding; a pair of capacitors, each comprising assaass a flat plate of insulating material provided with a pair of conductive electrodes disposed on opposite surfaces of said insulating plate, supported by said frame with the insulating plate thereof disposed in parallel relation to said frame; means electrically coupling one of said condensers in parallel with said primary winding; and means electrically coupling the other of said condensers in parallel with said secondary winding.

7. An intermediate frequency transformer comprising: a supporting frame; a first ferromagnetic core structure comprising a core member, having a non-linear principal axis, fixed to said frame, the axial extremities of said core member being disposed along a line parallel to a first predetermined axis, and further comprising a tuning member movable along said axis and bridged between said extremities of said core member to constitute therewith a single-loop magnetic circuit having an air gap of variable sizes determined by the relative positions of said members; a primary winding carried on said core member; a second ferromagnetic core structure comprising a core member having a non-linear principal axis, fixed to said frame, the axial extremities of said last-mentioned core member being disposed along a line parallel to a second predetermined axis, and further comprising a tuning member of linear configuration movable along said second axis and bridged between said extremities of the last-mentioned core member to constitute therewith a single-loop magnetic circuit having an air gap of variable size determined by the relative positions of such members; a secondary winding carried on said last-mentioned core member and inductively coupled to said primary winding; a pair of capacitors, each comprising a flat plate of insulating material provided with a pair of conductive electrodes disposed on opposite surfaces of said insulating plate, supported by said frame with the insulating plate thereof disposed in parallel relation to said frame; means electrically coupling one of said condensers in parallel with said primary winding; and means electrically coupling the other of said condensers in parallel with said secondary Winding.

8. An intermediate-frequency transformer comprising: a supporting frame; a first single-loop ferromagnetic core structure comprising a core member, having a non-linear principal axis, fixed to said frame, the axial extremities of said core member being disposed along a line parallel to a first predetermined axis, and further comprising a tuning member supported for movement along said first axis contiguous to said extremities of said core member; a primary Winding carried on said core member; a second single-loop ferromagnetic core structure comprising a core member, having a non-linear principal axis, fixed to said frame, the axial extremities of said last-mentioned core member being disposed along a line parallel to a second predetermined axis, and further comprising a tuning member supported for movement along said second axis contiguous to said extremities of said last-mentioned core member; and a secondary winding carried on said last-mentioned core member and inductively coupled to said primary winding.

References Cited in the file of this patent UNITED STATES PATENTS 865,985 Bains Sept. 17, 1907 1,875,968 Weeber Sept. 6, 1932 2,158,613 Loughlin May 16, 1939 2,283,924 Harvey May 26, 1942 2,388,848 Howe Nov. 13, 1945 2,544,508 Mackey Mar. 6, 1951 2,547,948 Kornei Apr. 10, 1951 2,550,244 Gusdorf et a1 Apr. 24, 1951 2,575,099 Crowley Nov. 13, 1951 2,617,092 Schlawin Nov. 4, 1952 2,631,192 Wallin Mar. 10, 1953 2,675,526 Friberg Apr. 13, 1954 2,748,357 Garcia May 29, 1956 2,774,057 Jones Dec. 11, 1956 

